In the process of making paper stock, ingredients such as paper pulp and water are fed at a controlled rate into a stock mixing tank, often called a “machine chest.” After initial mixing in the unrefined stock chest, the paper pulp and water mixture is fed into a second vessel called a “primary machine chest.” A higher consistency of paper pulp output from the primary machine chest (by weight percent) tends to lead to a more efficient papermaking process (e.g., less paper stock required for a given minimum paper thickness).
Consistency of paper pulp weight percentage in paper stock has traditionally been achieved by using large mixing vessels and adding water injection control loops and water bypass loops into the paper pulp and water mixing system. Also, in many typical paper pulp mixing systems, a single impeller is located at the vessel bottom, near the outlet, such that there is only a single mixing zone. This configuration may be prone to channeling or insufficient mixing.
Some paper pulp mixing systems use multiple impellers. However, the inventors theorized that when impellers are spaced too close to each other or are operated at a high speed, they may create a single large mixing zone. This results in a “short circuiting” tank dynamic behavior, where fresh material added to the top of the stock chest is quickly pulled downward by the impellers towards the outlet, without much mixing. As in channeling, the short circuiting problem has the disadvantages of a limited mixing zone (some of the paper pulp towards the vessel sidewalls is not being mixed, producing “stagnant zones”) and inconsistency of paper pulp weight percentage at the outlet (because the newly-added paper pulp is forced towards the outlet before sufficient mixing can be achieved). This system dynamic can only have limited improvement by using a control loop feedback system, because there is too short of a time delay between the inlet disturbance and the outlet signal.
The inventors theorize that in order to prevent short circuiting behavior, some paper pulp mixing systems use multiple impellers that are spaced too far apart from each other or are operated at too low of a speed, which may create separate mixing and dynamic behavior that forms “caverns” and stagnant zones. The caverns are the separate mixing zones that mix a relatively small portion of the paper pulp in the stock chest. This leads to inadequate mixing of the paper pulp and water, and it results in poor paper pulp weight percentage consistency at the outlet.
Paper pulp often contains more than 10% air (by volume), which is bound in the fiber network, primarily in the form of small bubbles. Excessive entrapped air in the paper pulp is undesirable in the paper-making process.
This description of the background summarizes some observations of the prior art. However, the disclosure identified as the theorizing of the inventors is not intended to be an admission that the observations are part of the prior art. Further, the present invention is not limited to possessing all of these characteristics that constitute an advance over the prior art nor is the present invention limited to possessing all the solutions to the problems of the prior art.